In information apparatuses such as a computer, a DRAM (Dynamic RAM) with which a high-speed operation and high-density recording are possible is widely used as a RAM (Random Access Memory). However, since the DRAM is a volatile memory in which information is deleted when power is turned off, a high-speed, high-density, and large-capacity nonvolatile memory that is capable of holding information even when power is shut off and is indispensable for reducing power consumption of an apparatus is strongly demanded.
A flash memory and the like is put to practical use as the nonvolatile memory, but a magnetic memory that uses a magnetoresistance effect is attracting attention and being developed as a high-speed, large-capacity, and low-power-consumption nonvolatile memory in recent years and is thus being developed. For example, an MRAM (Magnetic RAM) constituted of a magnetic memory device that uses a TMR (Tunnel Magnetoresistance) effect, that is, an MTJ device and onto which information is recorded by inverting a magnetization direction of a recording layer by a magnetic field induced by a current is put to practical use (e.g., MR2A16 (product name) available from Freescale Semiconductor, Inc.).
FIG. 9(a) is an explanatory diagram showing a basic structure of an MTJ device and an operation of reading out recorded information. As shown in FIG. 9(a), an MTJ device 100 has a structure in which a tunnel insulation layer 104 as a thin nonmagnetic insulation layer is interposed between a recording layer 105 and a magnetization reference layer 103 as two ferromagnetic layers, that is, a so-called MTJ (Magnetic Tunnel Junction). The recording layer 105 is constituted of a ferromagnetic conductor having uniaxial magnetic anisotropy and is capable of changing a magnetization direction by an external operation and holding the magnetization direction as information. For example, which of “parallel” and “nonparallel” the magnetization direction is with respect to a magnetization direction of the magnetization reference layer 103 is stored as information of “0” and “1”.
For reading out information from the MTJ device 100, a TMR effect in which a resistance value with respect to a tunnel current that flows between the recording layer 105 and the magnetization reference layer 103 via the tunnel insulation layer 104 is changed by a relative difference in the magnetization directions between the two magnetic layers described above is used. The resistance value used herein takes a minimum value when the magnetization direction of the recording layer 105 and that of the magnetization reference layer 103 are parallel and takes a maximum value when the magnetization directions are nonparallel.
FIG. 9(b) is a partial perspective view showing an example of a memory cell structure of an MRAM constituted of the MTJ device 100. In the MRAM, a memory cell corresponding to 1 bit is formed by arranging word lines as row wirings and bit lines as column wirings in a matrix and arranging the MTJ device 100 at intersections thereof.
At an upper portion of the memory cell, a write bit line 122 and a read bit line 123 are provided with an interlayer insulation film interposed therebetween, the MTJ device 100 is provided below the read bit line 123 while being in contact therewith, and a write word line 121 is provided below an extraction electrode layer 106 of the MTJ device 100 with an insulation layer interposed therebetween.
On the other hand, at a lower portion of the memory cell, a MOS (Metal Oxide Semiconductor)-type field-effect transistor is provided on a semiconductor substrate 111 such as a silicon substrate as a selection transistor 110 for selecting a memory cell at a time of a read operation. A gate electrode 115 of the transistor 110 is formed as a band connecting the cells and also functions as a read word line. Moreover, a source area 114 is connected to the extraction electrode layer 106 of the MTJ device 100 via a read connection plug 107, and a drain area 116 is connected to a sense line 124 as a read row wiring.
In the MRAM having the structure as described above, write (recording) of information to the MTJ device 100 of a desired memory cell is carried out by causing write currents to flow through the write word line 121 and the write bit line 122 of a row and column included in the memory cell and causing a synthetic magnetic field of magnetic fields generated by those currents at intersections of the two write wirings. By the synthetic magnetic field, the recording layer 105 of the MTJ device 100 of the desired memory cell is magnetized in a predetermined magnetization direction, that is, a direction that is “parallel” or “nonparallel” to the magnetization direction of the magnetization reference layer 103, to thus write (record) information.
Further, for reading out information from the MTJ device 100, a selection signal is applied to the gate electrode 115 as a read word line of a row included in a desired memory cell to put the entire selection transistor 110 of that row to an ON (conductive) state. At the same time, a read voltage is applied between the read bit line 123 of a column included in the desired memory cell and the sense line 124. As a result, only a desired memory cell is selected, and a difference of the magnetization direction of the recording layer 105 of the MTJ device 100 is detected as a difference in a level of a tunnel current flowing through the MTJ device 100 using the TMR effect. The tunnel current is extracted to a peripheral circuit (not shown) from the sense line 124 and measured.
It is reported that since the TMR-type MRAM is a nonvolatile memory from which information is read out using a magnetoresistance effect that is based on a spin-dependent conduction phenomenon unique to a nanomagnet and rewrite is performed by inverting a magnetization direction, rewrite can be practically performed an unlimited number of times and a high-speed access time is realized (see, for example, “ISSCC Digest of Technical Papers”, R. Scheuerlein et al., pp. 128-129, February 2000).
However, in the MRAM on which write is performed using a current magnetic field, it is necessary to cause a large current (e.g., about several mA) to flow for rewrite, with the result that power consumption increases. Moreover, since a write wiring becomes thin while a current required for rewrite tends to increase along with a miniaturization of an MTJ device, it becomes difficult to cause a sufficient current for rewrite to flow. Further, with a progress of a high integration, a possibility of information being erroneously written in a different adjacent memory cell increases. Furthermore, since both a write wiring and a read wiring are required, a structure becomes complex. As a result, an increase in a density and capacity of the MRAM on which write is performed using a current magnetic field is limited.
In this regard, a magnetic memory device that uses a magnetization inversion by a spin injection for write is attracting attention as a device that writes (records) information onto a recording layer of a magnetic memory device based on a different principle. The spin injection is an operation of causing a current to flow through a ferromagnetic conductive layer (magnetization reference layer) whose magnetization direction is fixed to generate a current constituted of an electron ensemble whose spin direction is polarized in one direction (spin-polarized current) and injecting the current into a magnetic conductive layer (recording layer) that is capable of changing its magnetization direction. By such an operation, a force that acts to make the magnetization direction of the recording layer and that of the magnetization reference layer coincide (torque) is caused by a mutual action between a spin-polarized electron and an electron of a magnetic body constituting the recording layer at a time the spin-polarized current flows through the recording layer. Thus, by causing the spin-polarized current having a current density equal to or larger than a certain threshold value to flow, the magnetization direction of the recording layer can be inverted (see, for example, Patent Document 1 and Non-patent Document 1 to be described later).
FIG. 10 is a partial perspective view showing an example of a structure of an MRAM shown in Patent Document 2 to be described later, that is constituted of an MTJ device whose magnetization direction is inverted by a spin injection (hereinafter, referred to as spin injection MTJ device) and uses a magnetization inversion by the spin injection (hereinafter, referred to as spin torque MRAM). In the spin torque MRAM, word lines 215 as row wirings and bit lines 218 as column wirings are arranged in a matrix, and one spin injection MTJ device 220 is arranged at each intersection, to thus form a memory cell corresponding to 1 bit. FIG. 10 shows four memory cells.
On a semiconductor substrate 211 at a lower portion, a selection transistor 210 to be described later is formed in each memory cell, and the word line 215 also functions as a gate electrode of the selection transistor 210. Moreover, a drain area 216 is commonly formed for the selection transistors shown on both sides in the figure, and a row wiring 219 is connected to the drain area 216.
FIG. 11 is a partial cross-sectional diagram showing a memory cell structure of the spin torque MRAM. At a center portion of the memory cell, the spin injection MTJ device 220 is formed by laminating, in the stated order from the bottom layer, a base layer 201, an antiferromagnetic layer 202, a magnetization fixing layer 203a, an intermediate layer 203b, a magnetization reference layer 203c, a tunnel insulation layer 204, a recording layer 205, and a protective layer 206. The layer structure of the spin injection MTJ device 220 is basically the same as that of the normal MTJ device 100.
The magnetization fixing layer 203a, the intermediate layer 203b, and the magnetization reference layer 203c are laminated on the antiferromagnetic layer 20202 and constitute a fixed magnetization layer as a whole. A magnetization direction of the magnetization fixing layer 203a constituted of a ferromagnetic conductor is fixed by the antiferromagnetic layer 20202. The magnetization reference layer 203c similarly constituted of a ferromagnetic conductor forms an antiferromagnetic bond with the magnetization fixing layer 203a via the intermediate layer 203b as a nonmagnetic layer. As a result, a magnetization direction of the magnetization reference layer 203c is fixed in a direction opposite to the magnetization direction of the magnetization fixing layer 203a. In the example shown in FIG. 11, the magnetization direction of the magnetization fixing layer 203a is fixed in a left-hand direction, and the magnetization direction of the magnetization reference layer 203c is fixed in a right-hand direction.
Since a sensitivity of the fixed magnetization layer with respect to an external magnetic field can be lowered when the fixed magnetization layer has the multilayer ferrimagnetic structure described above, it is possible to suppress a magnetization fluctuation of the fixed magnetization layer due to the external magnetic field and improve a stability of the MTJ device. In addition, since magnetic fluxes that leak out from the magnetization fixing layer 203a and the magnetization reference layer 203c cancel each other out, the magnetic flux that leaks out from the fixed magnetization layer can be suppressed to a minimum level by adjusting film thicknesses of those layers.
The recording layer 5 is constituted of a ferromagnetic conductor having uniaxial magnetic anisotropy and capable of changing a magnetization direction by an external operation and holding the magnetization direction as information. For example, which of “parallel” and “nonparallel” the magnetization direction is with respect to the magnetization direction of the magnetization reference layer 203c is stored as information of “0” and “1”. The tunnel insulation layer 204 as a thin nonmagnetic insulation layer is interposed between the magnetization reference layer 203c and the recording layer 205, and the magnetization reference layer 203c, the tunnel insulation layer 204, and the recording layer 205 form an MTJ (Magnetic Tunnel Junction).
On the other hand, at a lower portion of the memory cell, a MOS-type field-effect transistor constituted of a gate insulation film 212, a source electrode 213, a source area 214, a gate electrode 215, the drain area 216, and drain electrodes 217 is provided as the selection transistor 210 for selecting a memory cell in a device-isolated well area 211a of the semiconductor substrate 211 such as a silicon substrate.
As described above, the gate electrode 215 of the selection transistor 210 is formed as a band connecting the cells and also functions as a word line as a first row wiring. Moreover, the drain electrodes 217 are connected to the row wiring 219 as a second row wiring, and the source electrode 213 is connected to the base layer 201 of the spin injection MTJ device 220 via a connection plug 207. On the other hand, the protective layer 206 of the spin injection MTJ device 220 is connected to the bit line 218 as the column wiring provided at the upper portion of the memory cell.
For recording information onto the spin injection MTJ device 220 of a desired memory cell, a selection signal is applied to the word line 215 of a row included in the desired memory cell to put the entire selection transistor 210 of that row to an ON (conductive) state. At the same time, a write voltage is applied between the bit line 218 of a column included in the desired memory cell and the row wiring 219. As a result, a desired memory cell is selected, a spin-polarized current flows through the recording layer 205 of the spin injection MTJ device 220, and the recording layer 205 is magnetized in a predetermined magnetization direction, to thus record information.
At this time, the magnetization direction of the magnetization reference layer 203c of the spin injection MTJ device 220 is first “nonparallel” to the magnetization direction of the recording layer 205. When inverting the magnetization direction of the recording layer 205 to be “parallel” to the magnetization direction of the magnetization reference layer 203c by write, a write current having a current density equal to or larger than a threshold value is caused to flow from the recording layer 205 to the magnetization reference layer 203c as shown in FIG. 11. Accordingly, a spin-polarized electron stream having an electron density equal to or larger than a threshold value flows substantively from the magnetization reference layer 203c to the recording layer 205 to thus cause a magnetization inversion.
Conversely, when inverting the magnetization direction of the magnetization reference layer 203c that is “parallel” to the magnetization direction of the recording layer 205 to be “nonparallel”, a write current having a current density equal to or larger than a threshold value is caused to flow in the opposite direction, that is, from the magnetization reference layer 203c to the recording layer 205 so that an electron stream having an electron density equal to or larger than a threshold value flows substantively from the recording layer 205 to the magnetization reference layer 203c. 
Further, information is read out from the spin injection MTJ device 220 using the TMR effect as in the case of the MTJ device 100. While write and read with respect to the spin injection MTJ device 220 both use a mutual action between the electron of the recording layer 205 and the spin-polarized current that flows through the recording layer 205, read is performed in an area where the current density of the spin-polarized current is small, whereas write is performed in an area where the current density of the spin-polarized current is large and exceeds a threshold value.
Whether the magnetization inversion by the spin injection can be performed depends on the current density of the spin-polarized current. Thus, as a volume of the recording layer decreases in the spin injection MTJ device 220, the magnetization inversion can be performed with a smaller current in proportion to the volume (see Non-patent Document 1). Further, since information is written to a memory cell selected by the selection transistor 210, there is no fear of information being erroneously written to a different adjacent cell unlike write that uses a current magnetic field. Furthermore, since most of the wirings can be shared in write and read, a mechanism can be simplified. Moreover, since an influence of a shape of a magnetic body is smaller than that in the case of the write that uses a magnetic field, a yield ratio in production is apt to increase. Based on those points, the spin torque MRAM is more suited for realizing a miniaturization and an increase in the density and capacity than the MRAM on which write is performed using a current magnetic field.
However, a different problem arises when write (recording) is performed using the selection transistor 210. Specifically, a current that can be caused to flow through the spin injection MTJ device 220 at the time of write is limited by a current that can be caused to flow through the selection transistor 210 (saturation current of transistor). In general, since the saturation current of the transistor becomes smaller as a gate width or gate length of the transistor becomes smaller, a miniaturization of the selection transistor 210 is limited for securing a write current to the spin injection MTJ device 220. Therefore, for miniaturizing the selection transistor 210 as much as possible and maximumly increasing the density and capacity of the spin torque MRAM, it is necessary to reduce a threshold value of a write current as much as possible.
Moreover, also for preventing an insulation breakdown of the tunnel insulation layer 204 from occurring, it is necessary to reduce a threshold value of a write current. In addition, also for reducing power consumption of the MRAM, it is necessary to reduce a threshold value of a write current as much as possible.
It is phenomenologically shown that a threshold value of a current required for the magnetization inversion by a spin injection is proportional to a spin brake constant α, a square of a saturated magnetization amount Ms, and a volume V of the recording layer 205 and inversely proportional to a spin injection efficiency η. Therefore, by appropriately selecting them, a threshold value of a current required for the magnetization inversion can be reduced.
On the other hand, however, for the spin injection MTJ device 220 to be a reliable memory device, it is necessary to secure memory holding characteristics (thermal stability of magnetization) of the recording layer 205 so that the magnetization direction does not change by a thermal motion. The thermal stability is proportional to the saturated magnetization amount Ms and the volume V of the recording layer 205.
The saturated magnetization amount Ms and the volume V of the recording layer 205 relate to both a threshold value of a current required for the magnetization inversion and the thermal stability and are in a tradeoff relationship in which, by reducing a threshold value of a current required for the magnetization inversion by reducing those factors, the thermal stability is also lowered.
Therefore, it is necessary to improve, for reducing a threshold value of a current required for the magnetization inversion, mainly the spin injection efficiency η while carefully taking a balance with a securement of the thermal stability. The inventors of the present invention have vigorously developed an MTJ material that is capable of realizing both a reduction in a threshold value of a current density required for a magnetization inversion and a securement of memory holding characteristics (thermal stability) so that a spin torque MRAM becomes a competitive memory as compared to other memories (see Japanese Patent Application Laid-open No. 2006-165265, Japanese Patent Application Laid-open No. 2007-103471, Japanese Patent Application Laid-open No. 2007-48790, Patent Document 2, Japanese Patent Application No. 2006-350113, etc.). As a result, the MTJ material is now close to being realized.    Patent Document 1: Japanese Patent Application Laid-open No. 2003-17782 (p. 6 and 7, FIG. 2)    Patent Document 2: Japanese Patent Application Laid-open No. 2007-287923 (p. 7-15, FIG. 2)    Non-patent Document 1: “Appl. Phys. Lett.”, F. J. Albert et al., Vol. 77 (2002), p. 3809